1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a bias-T apparatus and its center conductor for providing radio frequency signals and power source to outdoor equipment of a Base Transceiver Station (BTS) in a mobile communication system.
2. Description of the Related Art
In general, ground equipment in a tower-top BTS either employs a pre-amplifier or enables an upward/downward frequency converter to have an additional amplifying function, in order to compensate for loss of signal or power in an inter facility link cable (IFL) connected to a remote radio frequency (RF) unit located in outdoor equipment (an antenna tower) of the BTS. In addition, the ground equipment also employs a bias-T apparatus or line amplifier, in order to supply electric power to the remote RF unit located in the antenna tower.
FIG. 1 is a block diagram of one example of conventional tower-top BTSs. Referring to FIG. 1, a conventional BTS includes a ground BTS 100 constructed on the ground and an antenna tower 114 constructed outdoor in order to improve the transmission/reception level of radio frequency (RF) signals.
Hereinafter, the construction of the ground BTS 100 will be referred first. A control/interfacing unit 102 performs signal matching with a base station controller (BSC) for controlling the BTS and generally controls the operation of the BTS. A digital processor 104 CDMA-modulates and -demodulates forward and backward signals according to the control of the control/interfacing unit 102. An upward/downward frequency converter 106 upward-converts the forward signal into a signal of a CDMA wireless band and downward-converts the backward signal into an intermediate frequency signal used in the digital processor 104. A pre-amplifier 108 compensates for line loss of the signal outputted from the upward/downward frequency converter 106 and amplifies the signal to meet input requirements for RF processing of the signal by the antenna tower 114. A bias-T circuit 110 synthesizes power and the output signal of the pre-amplifier 108 and transmits the synthesized signal through an IFL cable to the antenna tower 114, and a bias-T circuit 112 receives a signal from the antenna tower 114 and transmits the signal to the upward/downward frequency converter 106.
Next, the antenna tower 114 will be described, which includes a main amplifier 116, a low noise amplifier 124, and antennas 118 and 120. The main amplifier 116 amplifies power of the signal from the bias-T circuit 110 up to a level which meets a standard required for a forward radio link to a mobile subscriber terminal. The low noise amplifier 124 amplifies with minimum noise the signal received from the antenna 120 and then transmits the amplified signal to the bias-T circuit 112.
FIG. 2 is a circuit diagram of the bias-T circuit shown in FIG. 1. In the bias-T circuit as shown in FIG. 2, when the signal outputted from the pre-amplifier 108 is inputted through a signal input node 201, the signal is outputted to a signal output node 202 through a capacitor C1 204 but is not outputted to another node 203 through which direct current is applied. It is because the inputted signal is a radio frequency signal which causes the node 203 to have an infinite impedance due to an inductor L1 205. Further, direct current (DC) power is inputted through the power input node 201 and the inductor 205. Therefore, the output signal of the pre-amplifier 108 and the DC power are synthesized through the bias-T circuit 110 and are then transmitted to the antenna tower 114.
Further, the bias-T circuit 112 processes the backward signal in the same way, thereby transmitting the backward signal to the upward/downward frequency converter 106.
There are several ways of constructing the capacitor C1 204. First, a chip capacitor may be employed. However, in this case, a portion between an input node and a transmission line causes the assembling of the capacitor to be more complicated and difficult.
Second, two electrode plates (that is, a center conductor) constituting the capacitor C1 are installed at a portion between the input node and the transmission line while being spaced a predetermined gap from each other. In general, a capacitor has a capacitance which is proportional to the area size of the electrode plates and inversely proportional to the distance between the electrode plates. However, recent electronic appliances and their elements tend to be lighter, thinner, shorter, and smaller, thereby causing it difficult to enlarge the area of the electrode plates in order to increase the capacitance since the area of the electrode plates has a large influence on the size of the capacitor. Therefore, a method of reducing the gap between the electrode plates is usually employed. However, there is a limitation in reducing the gap between the electrode plates, and thus there is a limitation in increasing the capacitance of the capacitor.